Examples of what are known in digital X ray imaging are image intensifier camera systems based on television or CCD cameras, storage film systems with an integrated or external readout unit, systems with optical coupling of the converter foil to CCD cameras or CMOS chips, selenium-based detectors with electrostatic readout, and X ray detectors, in particular flat image detectors, with active readout matrices with direct or indirect conversion of the X radiation.
Such a detector is based on an active readout matrix, for example made from amorphous silicon (a-Si), that is precoated with an X ray converter layer or scintillator layer, for example made from cesium iodide (CsI). The incident X radiation is firstly converted into visible light in the scintillator layer. The active matrix is subdivided into a multiplicity of pixel readout units with photodiodes, which photodiodes in turn convert this light into electric charge and store it in a spatially resolved fashion on their electrode.
An active readout matrix is likewise used in the case of a so called directly converting flat image detector. However, this active readout matrix is arranged downstream of a converter layer, for example made from selenium, in which the incident X radiation is converted directly into electric charge. This charge is then stored, in turn, in the pixel readout units.
The stored electric charge is subsequently read out via an active switching element, for example a transistor, of the pixel readout unit with the aid of dedicated electronics, in order subsequently to be further processed.
The duration of the readout process together with the duration of the X ray time window, that is to say the duration of the application of the X radiation, restricts the imaging rate. The imaging rate is defined as the number of X ray pictures per time unit. Particularly in the case of dynamic X ray applications that include a sequence of many X ray pictures, however, both wide X ray time windows and high imaging rates are particularly important. Angiography and fluoroscopy, for example, are counted among the dynamic X ray applications.